WHAT ARE METEORITES?

WHAT ARE METEORITES?

WHAT ARE METEORITES?

A meteorite is a solid piece of debris from an object, such as a
comet, asteroid, or meteoroid, that originates in outer space and
survives its passage through the atmosphere to reach the surface
of a planet or moon.

A meteorite is a solid piece of debris from an object, such as a
comet, asteroid, or meteoroid, that originates in outer space and
survives its passage through the atmosphere to reach the surface
of a planet or moon.

When the object enters the atmosphere, various factors like friction,
pressure, and chemical interactions with the atmospheric gases cause
it to heat up and radiate that energy. It then becomes a meteor and
forms a fireball, also known as a shooting star or falling star;
astronomers call the brightest examples "bolides." Meteorites vary
greatly in size. For geologists, a bolide is a meteorite large enough
to create a crater.

Meteorites have traditionally been divided into three broad categories:
stony meteorites are rocks, mainly composed of silicate minerals;
iron meteorites that are largely composed of metallic iron-nickel;
and, stony-iron meteorites that contain large amounts of both metallic
and rocky material. Modern classification schemes divide meteorites
into groups according to their structure, chemical and isotopic
composition and mineralogy.

TYPES OF METEORITES

TYPES OF METEORITES

STONY METEORITES

Stony meteorites are by far the most common. More than 95% of meteorites
observed to fall to Earth are stony. They can be divided into chondrites
and achondrites. Both types are composed mostly of silicate minerals, but
the great majority also contain metallic iron in small-scattered grains.

Chondrites are named for their most prominent feature - millimeter-sized
spherical bodies called chondrules. These chondrules (from the Greek for
small sphere) formed 4.5 billion years ago in the Solar Nebula - the cloud
of gas and dust from which the Sun, planets, asteroids, and comets formed.
Chondrules are not found in terrestrial rocks. These chondrules, along with
small mineral grains, accreted to form asteroids during the birth of the Solar
System. Chondrites are, by far, the most abundant type of stony meteorite.

Less common, comprising only a few percent of all meteorites, are achondrites.
These are also stony meteorites composed primarily of silicates, but these
meteorites have experienced familiar geologic processes of melting and
differentiation - although these happened long ago. Most achondrites formed on
asteroids during the birth of the Solar System, but a small number formed on
Mars and the Moon.

STONY-IRON METEORITES

Stony-iron meteorites, contain about equal proportions of metal and silicate
material, and are rare (less than 2% of all known meteorites). Stony-iron
meteorites form in places where metal and silicate are mixed.

One type of stony-iron are pallasites - rocks composed of a network of iron-nickel
metal surrounding a greenish, silicate mineral called olivine. Pallasites probably
form when the olivine-rich mantle of an asteroid mixes with the metallic core.
Mesosiderites are mixtures of iron-nickel metal and basalt and probably formed
by the collision of two asteroids.

IRON METEORITES

Iron meteorites are really composed of iron and nickel and are extremely dense.
They are pieces of the cores of asteroids. Early in Solar System history, asteroids
melted and the dense iron-nickel metal sank to the center to form a core - much like
the Earth has a core. Iron meteorites are the samples of the cores of ancient worlds.

While they are rare among meteorites seen to fall to Earth (only a few percent),
they are among the most common type of meteorites in our collections, because they
can be recognized long after their fall, are very different from Earth rocks,
and are resistant to weathering.

One of the most distinguishing features of meteorites is the presence of the
Widmanstatten pattern - the distinctive series of bands in geometric patterns.
This pattern is created by the intergrowth of two different iron-nickel minerals
formed during very slow cooling (a few degrees every million years) in the core of
the asteroid. The presence of nickel is a universal feature of iron meteorites.

AGE & ORIGINATION

AGE & ORIGINATION

Meteorites range in age. The oldest particles in a meteorite, calcium-aluminum-rich
inclusions from carbonaceous chondrites, have been dated at 4.56 billion years old.
Meteorites that originate from asteroids are all ~4.5 billion years old. Meteorites
that originate from the Moon range in age from 4.5 to 2.9 billion years old.
Meteorites that originate on Mars range in age from 4.5 billion years old to
200 million years old.

Most meteorites are believed to originate in the asteroid belt between Mars and Jupiter,
and were formed early in the history of the Solar System ~4.56 billion years ago.
These fragments of asteroids were either knocked out of their orbit of the Sun, and
into Earth-crossing orbits, through collisions with other objects, or through the
interaction of gravitational forces exerted by the Sun and Jupiter.